Gyroscope float



United States Patent Ofice 3,442,142 Patented May 6, 1969 3,442,142 GYROSCOPE FLOAT John Thomas Gresham, Skillman, N.J., assignor to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed May 10, 1966, Ser. No. 548,853 Int. Cl. G01c 19/04 US. Cl. 74-5.5 3 Claims This invention relates to an improvement in floated control instruments and particularly in gyroscopes using lowviscosity flotation fluids and, more particularly, to the use of 1,2,3,4-tetrabromohexafluorobutane in such gyroscopes.

A floated gyroscope is one in which the rotor is encased in a float. The density of the flotation fluid is chosen to match the density of the float. The float is thus supported for rotation about its outward axis on. practically frictionless gimbal bearings. The gyro fluid also protects the assembly from shock, provides a damping function and a heat transfer function to dissipate heat which may be developed by the rotor. Where the damping effect on the precession of the float is largely developed by the fluid itelf, the fl-uid is generally selected to have a high viscosity. However, Where damping is not desired, or where electronic or orifice damping is resorted to, low viscosity liquids are preferred.

In addition to the above use in a single gyroscope, a gyro fluid may be used in a system in which one or more gyros, one or more rate gyros or accelerometers, and associated equipment are mounted on a platform which is floated in the fluid. Fluids for this application are usually dense, low viscosity liquids for flotation and heat transfer.

In addition to being useful in gyroscopes, 1,2,3,4-tetrabromohexafluorobutane can be used as a fluid in other control systems where an element is desirably floated or submerged in a high density fluid.

The liquids which are useful in gyroscopes must have certain critical physical characteristics. High density is required, at least about 1.6 g./ml. The higher the density, the more useful is the fluid, since this permits more inertia to be built into the rotor without higher loads on the gimbal bearing. A higher density fluid would allow size reduction at the same amount of inertia (and sensitivity of the gyro). A second requirement of such liquids is that the liquids show Newtonian behavior, i.e., one which is independent of the shearing rate. Preferably also the liquid should be relatively insensitive in viscosity to temperature variations. It must of course remain useful over the operational temperature range of the gyroscope, and should not solidify. Finally, it is essential that the liquid remain stable under the operating conditions of the gyroscope, and that it be non-corrosive to the metal of which the gyroscopes are ordinarily prepared.

It has been extremely difficult to find fluids which meet these requirements. Those in general use today are based on fluorocarbons because of the known stabilizing effect of large numbers of fluorine substituents on halogenated carbon compounds. The high viscosity fluids are generally telomers of CFCI=CF and CFBr=CF which have densities of about 1.9 and 2.4 g./ml., respectively. Where low viscosity liquids are used, low molecular weight ends of the telomerization product of the olefins are sometimes selected; a volatile fluorinated cycloether, having the empirical formula, C F O, and a density of 1.7602 g./ ml. at 25 C., is also extensively used.

I have now discovered that the aforedelineated require- -ments for a gyro flotation are embodied to an usual extent in the exhaustively halogenated hydrocarbon, 1,2,3,

4-tetrabromohexafluorobutanc. The compound exhibits a suitable viscosity-about centipoises-and is exceedingly stable and inert. However, the most significant prop erty of the compound is its exceptionally high density- 2.58 g./ml. As a consequence it is idealas a gyro fluid thereby making it possible to construct floated instruments having greater sensitivity than heretofore realizable using similar fluids.

1,2,3,4-tetrabromohexafluorob-utane is a known compound the description and preparation of which can be found in the chemical literature. It is commonly obtained by brominating hexafluoro-1,3-butadiene, a commercially available chemical intermediate. The product is normally isolated by distillation. A purity of at least 99.0% is required for use as a gyro fluid. Details of the preparations are set forth in the following example.

EXAMPLE In a two liter flask equipped with a Dry Ice condenser, a gas dispersion tube and a stirrer is charged a mixture of 168 g. of bromine (5% excess) in methylene chloride. One hundred sixty-two grams of hexafluoro-1,3-butadiene (one mole) is slowly bubbled into the solution while the mixture is irradiated with ultraviolet light. After the addition the reaction mixture is heated and further irradiated until no further reaction takes place. The cooled reaction mixture is washed with sodium thiosulfate, then water, dried, and distilled, collecting the high boiling fraction. This fraction is redistilled on a spinning band column having about 40 theoretical plates. The fraction boiling at 105 C./40 mm. is pure 1,2,3,4-tetrabromohexafluorobutane, (1 2.588, n 1.464.

Purified 1,2,3,4-tetrabromohexafluorobutane is evaluated for corrosiveness using a test designed and developed by Frederick S. Bacon Laboratories (192 Pleasant Street, Watertown 72, Massachusetts). The test is designed to measure the relative corrosiveness of fluids by comparing the quantity of lead removed from a lead-plated copper strip by a test fluid, under standard conditions of time and temperature. The test is a recognized method for evaluating the corrosiveness of flotation fluids used in gyros.

Bearing Corrosion Test Strips, Type C for Copper-Lead Bearings (Randall and Sons, 2512 Etna Street, Berkeley 4, California) having seven steps of lead in increasing thicknesses of 3, 7, 14, 24, 37, 60 and microinches plated on a copper carrier, are used. All test strips have been cut from a single master sheet.

About 30 g. of purified 1,2,3,4-tetrabromohexafluorobutane is placed in each of three Pyrex test tubes (300 by 17 mm.). One standard test strip of lead-plated copper is added to each tube, the tubes are blanketed with argon, partially evacuated and sealed with a stopper and placed in an oil bath heated to a constant temperature of 1401-01 C. for a period of 48 hours. At the end of that time the tubes are removed from the oil bath and the extent of removal of each layer of lead coating observed. In all three cases the two thinnest layers are removed completely. About one-half of the third layer, originally 14 microinches thick, is also removed. Two repetitions of the test give practically the same results. Corrosion is determined by the number of steps removed from the coated strips; removal of 2.5 strips by l,2,3,4-tetrabromohexafluorobutane demonstrates that it is relatively non-corro sive. A mildly corrosive fluid removes completely a strip 14 microinches thick, and a corrosive fluid removes completely a strip 37 microinches thick.

A similar corrosion test is run with strips of beryllium at 100 C. At the end of the 48-hour test, there is no discoloration of the fluid and no evidence of corrosion on the surface of the beryllium.

1,2,3,4-tetrabromohexafluorobutane is particularly useful in any kind of floated gyroscope where damping by high .viscosityvfluids. is not important. It is likewise useful in other control instruments where a high gravity, low viscosity fluid is needed. It has the great advantage that along with the lower viscosity, it is substantially higher in density than known materials with similar desirable stability and is moreover an excellent heat transfer medium. Moreover, it is easy to prepare from commercially available materials and it is economical to produce.

As will be apparent to those skilled in the art, numerous modifications and variations of the formulation of the gyro flotation fluids illustrated above may be made without departing from the spirit of this invention or the scope of the following claims.

What is claimed is:

1. A control system comprising a control element in i4, 1 operative contact with a high gravity fluid consisting of 1,2,3,4-tetrabromohexafiuorobutane.

2. A gyroscope comprising a gyroscope float suspended in 1,2,3,4-tetrabromohexafluorobutane.

3. A guidance system containing a platform suspended in 1,2,3,4-tetrabromohexafluorobutan, said platform having mounted thereon at' least one gyro and associated equipment;

Refereuces"Cite'tl UNITED STATES PATENTS F RED C. MATTERN JJRLI, Rrimai'y Examiner. I MANUEL ANTONAKAS, 'Assi. vtarit Examiner; 

2. A GYROSCOPE COMPRISING A GYROSCOPE FLOAT SUSPENDED IN 1,2,3,4-TETRABROMOHEXAFLUOROBUTANE. 